Hi Matt,
Only talking engine performance.....and a couple of points will probably give you a bit more longevity with your standard turbo as well.
As mentioned above and I will take it to the grave the single best improvement I have done on mine and completely noticable straight away was a decent dump pipe with a good high flow cat(not that I have one....at all )
The next thing you should look at especially with the coming summer months is a decent FMIC. There are two out there...Blitz from Just jap and the HDI. Also look at putting a Nismo thermostat in....will save the top tank on your radiator.
From what I have heard the suction pipe is also a very good upgrade (fitting on to my car atm while it's off the road)
As far as ECU's go the best piggyback for our car's is probably the Greddy but you will need to find a good tuner. You will more than likely find that you standard ECU will still fight with it though. There is the F-con but finding a tuner to do it is a problem...BD4's don't do it anymore...spoke to the guy yesterday. As for a replacement ECU, the Mines ECU....if you can source it go for it.....I've tried but to no avail....even if you get it how do you tune it? If you can get a Nismo or Impul it is probably the best bolt on upgrade and again you notice the difference straight away. Oh and if SAS ever get the reader unit back that would without a doubt be the best option as you will be able to tune the standard ECU.
As far as turbo's go Scott's is probably one of the best working upgrades. Not saying that I have given up on my turbo setup either....I just have a few more hoops to just through yet. Both of us have had our own headache's with our setups but I can tell you first hand Scott's goes like shit off a stick straight out of the whole basically no lag....oh and not saying mine doesn't go hard either......the Mexicans are just a bit ahead of us with development ATM
BTW the standard turbo will probably only ever get you about 180KW's ATW's....tops.